This invention relates to raster scanned imaging system and, more particularly, to methods and means for recovering expanded gray scale information from a quantized video signal in such an imaging system.
By definition, an original document (whether it be a printed or written page, a drawing, photograph, or anything else of like character) is a two dimensional, generally continuous pattern of reflectance. The image or information content of a document can be entered into various types of electronic processing systems by a one dimensional sampling process, known as "raster input scanning", whereby the information content of the document is serially converted or re-mapped into a one dimensional video signal. Moreover, "raster output scanning" is available for serially reconstructing a processed version of the document in response to the video signal. Indeed, raster input and output scanning are widespread use in commercially available facsimile systems, to name just one of the better known types of "raster scanned imaging system."
If the video signal is preserved in an analog form, the continuous or gray scale aspect of the original document is retained. However, if the video signal is quantized, there are unadvoidable quantization errors which inherently result in a loss of gray scale information. As will be appreciated, the image degrading effects of the gray scale loss are a function of the coarseness of the quantization process. Half-toning may, of course, be used to at least partially compensate for the loss of gray scale information, but the effectivity of that technique is a function of the resolution provided for the imaging system. Thus, there is a particularly serious problem in low resolution system which involve coarse quantization of the video signal. A case in point is provided by modern, high speed facsimile systems in which the video signal is digitized so that digital data compression techniques can be employed to achieve reduced document transmission times.